The invention relates to data compression in dependence on a compression parameter. Such data compression may be applied, for example, in coding a sequence of images in accordance with an image-coding standard defined by the Moving Pictures Experts Group (MPEG).
U.S. Pat. No. 5,680,483 (attorney""s docket PHF 94.510) describes a device for coding a sequence of pictures divided into successive groups of N pictures arranged in blocks. It comprises a channel for quantization and variable length coding of these blocks and a bit rate control sub-assembly. This assembly comprises, in series, a buffer memory and a device for modifying the quantization step. The latter device comprises several stages. A first stage allocates bits for each successive picture of the sequence of pictures. A second stage allocates bits for a current picture and the Nxe2x88x921 subsequent pictures. A third stage corrects the bit allocation for each new picture to be coded. A fourth stage modifies the quantization step as a function of the corrected bit allocation.
It is an object of the invention to provide data compression which yields a better quality.
The invention takes the following aspects into consideration. It is advantageous to compress data to such an extent that an amount of compressed data is obtained which can just be handled by an entity receiving the compressed data, for example a transmission channel or a storage medium or both. If the data is compressed to a greater extent, there is an unnecessary loss of data because the data-handling capacity of the receiving entity is not used to its full extent. If the data is compressed to a smaller extent, there will also be an unnecessary loss of data because the receiving entity will not be able to handle a certain portion of the compressed data and, consequently, this portion will be lost. Thus, the quality of the data compression, in terms of minimal loss of information, is substantially influenced by a compression parameter determining to which extent the data is compressed.
The background art cited applies the following principle for adjusting a compression parameter in MPEG video coding. At the beginning of a group of pictures, an initial value of the compression parameter is calculated. The initial value of the compression parameter corresponds to the amount of compressed data which should preferably be obtained by compressing the data representing the group of pictures. On the basis of experiences in compressing a previous group of pictures, typical intermediate results are defined in terms of amounts of compressed data. A typical intermediate result is defined for each successive picture in the group. For each picture, the initial compression parameter is adapted on the basis of a difference between the amount of compressed data actually produced thus far and the typical intermediate result. Thus, in the background art, a picture-by-picture compression plan is made at the beginning of the group of pictures, and the compression parameter is adjusted in dependence on a deviation from this plan.
In accordance with the invention, the following steps are repetitively carried out. On the basis of an initial value of the compression parameter, it is predicted which amount of compressed data will have been obtained at a future instant of time. The initial value is adapted on the basis of a difference between the predicted amount of data and a desired amount of data, so as to obtain an adapted value of the compression parameter. The adapted value of the compression parameter is applied until a subsequent prediction step followed by a subsequent adaptation step is carried out.
The invention provides an adjustment of the compression parameter which is directly related to the desired amount of compressed data instead of indirectly as in the background art. As a result, an amount of compressed data will be obtained which is generally closer to the desired amount of compressed data than in the background art. Thus, the invention allows data compression which matches more accurately the data handling capacity of an entity receiving the compressed data. Consequently, the invention yields a better quality in terms of loss of information.
The invention and additional features, which may be optionally used to implement the invention to advantage, are apparent from and will be elucidated with reference to the drawings described hereinafter.